Abstract

Ever since Pasteur's demonstration of the modifying effect of animal passage on the virulence of bacteria, this device has been regarded as of great importance in intensifying infective capacity. It is not often, however, that extensive parallel tests have been made of the power of a microorganism to produce infection in a species of animal which is its natural habitat and under conditions in which the normal as well as artificial portal of entry is employed, and the potency of the microorganism passed directly from host to host is closely contrasted with that of one merely artificially cultivated outside the body.

The experiments described in this paper were arranged to fulfill these conditions. The results show that with the particular strain of Bacillus festis caviæ used, successive animal passages do not modify infective capacity, or virulence. This finding is in harmony with the observations previously made from which it was concluded that the epidemic curve of mouse typhoid infection is explicable solely on the basis of bacillary distribution and host susceptibility.